Differential Mortality - Background
The strength of natural selection is directly related to the degree of differential mortality in the population. If the probability of survival for an individual is a function of the value of a particular trait, then that trait is subject to selection pressures (assuming, of course, that the trait exhibits heritable variation). Selection will not occur if the probability of mortality is the same for all individuals. This is true even if there is a high degree of mortality in the population.
The relationship between mortality and a trait's value need not be a linear one for selection to occur. The probability of mortality may increase with the trait value, decrease with the trait value, increase with extreme trait values, increase for intermediate trait values or density dependent; an infinite array of possibilities exist.